A radiotherapy radiation source may be provided by, for example, a radio-frequency (RF) waveguide which may accelerate electrons to a desired energy level, either to subsequently produce x-rays by arranging for the electrons to collide with a tungsten target, for example, or for direct use in electron therapy.
Our WO 2007/124760 describes existing VMAT therapies, in which the dose rate is used as a variable in order to match the maximum speed of the moving machine axes and therefore create the specified dose rate, expressed as mu/mm or mu/°.
Our WO 2011/050887 sets out that a drawback of previously-known VMAT therapies is the longer treatment required because only a limited range of dose rates was available. That document proposed a radiotherapeutic apparatus comprising a source of radiation able to issue pulses of radiation at a base frequency, and a control apparatus arranged to permit or suppress pulses according to a pulse pattern chosen to achieve a selected time-averaged power level, the chosen pulse pattern being one selected from a plurality of pulse patterns, the plurality including at least one pattern consisting of a train of pulses that includes, relative to the base frequency, at least every nth and every mth pulse, where n≠m and the nth and Mth pulse are non-coincident.
The pulse pattern generated by that radiotherapeutic apparatus is able to modulate the output dose rate much more precisely than in previous radiotherapeutic apparatus, and thus reduce treatment times.
However, the pulse pattern generated by that invention could, under certain circumstances, include several consecutive permitted pulses followed by several consecutive suppressed pulses. Example pulse train patterns resulting from the apparatus of our '887 are shown in FIG. 1, and that drawing is described in more detail below. In that drawing it can be seen that permitted and suppressed pulses are sometimes grouped into blocks of consecutive permitted pulses and suppressed pulses.